Neuropeptides/Biogenic Amines In Stress &Brain Ischemia

National Institute Of Mental Health

Investigators

Linked publications & trials

Abstract

Our laboratory studies the central functions of the hormone and brain modulator Angiotensin II and in particular its role in the regulation of the cerebral circulation and the reaction to stress. We have earlier discovered that pre-treating spontaneously (genetic) hypertensive rats (SHR) with an inhibitor of the peripheral, cerebrovascular and brain Angiotensin II AT1 receptors, candesartan, protected against subsequent brain ischemia resulting from occlusion of a major cerebral artery, reversed the hypertension-induced shifts in cerebral blood flow autoregulation, preserves blood flow above a critical threshold at the periphery of the ischemic zone, reversed the pathological cerebrovascular remodeling characteristic of hypertension, thus normalizing cerebrovascular compliance, and inhibits cerebrovascular inflammation. AT1 receptor antagonists are superior, in its therapeutic potential for brain ischemia and stroke, when compared to other anti-hypertension medications. To clarify the mechanisms of the beneficial effects of the AT1 receptor antagonists, we constructed a large database, with the use of Gene Chip Expression Analysis, of changes in gene expression in microvessels of hypertensive and normotensive rats, treated with the AT1 receptor antagonist or vehicle. We found alterations in gene expression of a number of systems, including the local cerebrovascular Renin-Angiotensin System, neurotransmitter systems and signal transduction factors, heat shock proteins and other markers of inflammation, transporter systems regulating the function of the blood brain barrier, and in many genes related to lipid and carbohydrate metabolism. We are currently carrying out experiments to confirm the most important findings. We have established that AT1 receptor blockade increases the expression of the AT2 receptor, an Angiotensin II receptor type that may counteract the growth-promoting and inflammatory effects of AT1 receptor stimulation. We believe that AT2 receptor stimulation is an additional factor in the protective effect of AT1 receptor antagonists. AT1 receptor antagonism down regulates the expression of heat shock protein genes and inflammatory markers, which are upregulated in microvessels from hypertensive rats, and restoring the equilibrium between eNOS and iNOS, enhancing endothelial NO production and vasodilatation. We are revealing important mechanisms explaining the protective effect of AT1 receptor antagonism. A role of AT1 receptor inhibition in the protection from brain ischemia and inflammation beyond its effects in blood pressure control begins to emerge. In addition, AT1 antagonists may to prevent or reverse inflammatory conditions of the brain unrelated to hypertension. Our studies continue with the analysis of specific groups of genes selected on the basis of carefully researched specific questions, and the results obtained with the microarrays are carefully controlled and confirmed by means of RT-PCR for gene expression, Western blot for protein expression and immunohistochemistry, to localize the regulated proteins to specific cell types. AT1 receptor antagonists can be proposed to be elective first class antihypertensive medications with additional properties to protect from brain ischemia and inflammation. Our laboratory has earlier demonstrated that in addition to its vasoconstrictive, pro-hypertensive, pro-ischemic properties, Angiotensin II is an important stress hormone, and that pretreatment with a peripheral and central AT1 receptor antagonist completely prevents the sympathoadrenal response to isolation stress, including a modulation of TH transcription through regulation of transcription factors and including an interaction with AT2 receptors. We asked the question whether AT1 receptor antagonism could prevent a stress-induced illness, and we found that blockade of AT1 receptors completely prevented the production of stress-induced gastric ulcers in the rat during cold-restraint. We then found that multiple mechanisms were responsible for this effect, including protection of local gastric blood flow, selective inhibition of the sympathetic response to stress, and anti-inflammatory effects including decrease in ICAM expression and neutrophil infiltration in the gastric mucosa. The anti-inflammatory effects of AT1 receptor antagonism were of great interest, and coincided with the anti-inflammatory effects earlier described in the brain vasculature. Prevention of gastric ulcer formation, predominantly through anti-inflammatory effects, is a new finding of potentially important clinical implications. Our experiments continue to clarify our initial findings that in rodents, AT1 receptor blockade has an anxiolytic effect, as determined by behavioral studies using the Plus Maze. We initially proposed that the anxiolytic effects could be related to modulation of brain CRH and GABAA receptor expression. We now demonstrate that pretreatment with AT1 receptor antagonists with central effects prevents the decrease in CRF1 and benzodiazepine binding resulting from isolation stress. Thus, central inhibition of AT1 receptors counteracts the stimulation not only of the hypothalamic CRF system but of the cortical CRF system as well. Preservation of normal benzodiazepine binding during stress can be interpreted as protection of the cortical GABAA system leading to decreased anxiety during stress. This may indicate that AT1 receptor antagonists may be considered as a possible novel class of anti-stress, anti-anxiety medications. Because these compounds are devoid of addictive properties, development of new compounds of this class may result in medications of great therapeutic potential. Our studies continue to clarify the molecular mechanisms of the regulatory effect of AT1 receptor antagonists on the GABAA and CRF systems. Preliminary observations indicate that AT1 receptor antagonists prevent anxiety by modulation of the expression of selective GABAA subunits. Additional experiments demonstrate that there is a complete Renin-Angiotensin System in adipose tissue and that treatment with AT1 antagonists improve insulin sensitivity and increase the levels of adiponectin, a hormone released by adipose tissue and exerting anti-inflammatory effects in the vasculature. Some of these effects occur because of the adipocyte differentiation and lipolysis induced by the AT1 receptor antagonists. Our observations begin to clarify some of the basic molecular mechanisms of the anti-inflammatory effect of AT1 receptor antagonists, and of the anti-diabetic properties of some of these compounds. In conclusion, our studies indicate that non-peptidic antagonists of the AT1 receptor with central effects may be considered among the drugs of choice in the treatment of cardiovascular disease and brain ischemia, protect against stress-related disorders, protect against diabetes, exert important peripheral and central anti-inflammatory effects, and may be useful compounds to develop effective and non-addictive anti-anxiety and anti-stress drugs. We are planning to propose the first clinical study to determine if such compounds could be effectively used in the clinic to treat anxiety and stress-related disorders, and we continue our efforts to further clarify their mechanisms of action. The study of the complementary effects of AT2 receptor agonism and antagonism is another important direction in our current search for new and effective compounds of psychiatric interest.